In certain laser processing processes, such as laser annealing and laser crystallization of amorphous silicon (Si) layers, it is necessary to project laser radiation onto a workpiece in the form of a line of light. In projecting such a line it is desirable to have the smallest possible beam dimension in the width direction of the line to maximize the intensity of radiation in the line for a given line length. A limit to this is a line that has a width that is about diffraction limited, i.e., a line that has a width about comparable with the wavelength of radiation being projected. In the aforementioned processes it is usual to scan a workpiece through a line of radiation in a direction perpendicular to the line.
Typically, the higher the intensity that can be achieved in a projected line the faster the workpiece can be scanned and the higher the throughput of the process will be. Clearly, for any given line width and length, increasing the power in the beam being projected is the only way to increase line intensity. Accordingly, there is great interest in scaling the output power of lasers that deliver radiation at wavelengths useful for such processing. A CW power or an average power (in a pulsed beam) of greater than 500 Watts (W) is desirable
In any given laser type, it is difficult to scale up laser power without sacrificing beam quality. Beam quality determines the width of a focal spot (or line) into which the beam can be projected. The quality is defined by a quantity M2, the definition of which is well known to those familiar with the art. A single-mode (TEM00) beam has an M2 of slightly greater than 1.0 and can be projected into a near-diffraction-limited spot.
Laser costs do not scale linearly with power when beam quality must be maintained. Because of this, a laser delivering in excess of 500 W while still having a reasonable beam quality can be prohibitively expensive for certain manufacturing processes. One way of avoiding this problem has been to substitute for a single, very-high-power laser a plurality of lasers of lower power but higher beam quality and combine the laser output beams, using suitable optics, to form the line of radiation. Clearly, as far as the lasers are concerned, the cost of the plurality lasers will be at most about the product of the number of lasers and the cost of one, i.e., a linear scaling. As the number of lasers increases, however, the cost of fabricating an assembly of beam aligning, beam-combining, and beam-homogenizing optics that can be maintained in alignment in a commercial production environment becomes a significant part of a system. This tends to put a limit on the amount of beams that can practically be combined.
There is a need for apparatus that can project a line of radiation having a width of only a few times (say five-times) the diffraction limit from a very-high-power, poor-quality beam. The apparatus preferably should not require a complicated optical system that is difficult to align or difficult to maintain in alignment.